The present invention relates to a method for loading the blade of a doctoring unit used for coating, e.g., a paper web.
The invention also concerns an assembly for loading a doctor blade of a coater apparatus.
In coating of a paper web, the coat is smoothed onto the web most commonly using a doctor blade. The coat mix can first be applied onto the web using a separate applicator apparatus, whereby the doctor blade is placed at a distance in the machine direction from the point of application, or alternatively, a coating machine called a short-dwell coater can be used in which an application chamber is situated immediately beside the doctor blade and the applied coat is smoothed immediately by the doctor blade.
The amount of coat mix adhering to the web being coated is adjusted by altering the loading of the doctor blade. When the blade loading is altered, the actual angle of the doctor blade to the web also changes, and the control of applied coat weight is temporarily unstable. This relates to the fact that the blade is contoured conformant with the web at the previously used blade tilt angle, and after the change of this angle, a certain time must elapse before the blade tip again wears conformant with the web. Changes in blade angle also cause other coat defects such as bleeding and uneven coat thickness. To avoid such defects, different types of systems have been developed suited for maintaining the blade angle as constant as possible irrespective of changes in blade loading.
A change in the doctor blade angle can be compensated for by either deflecting the blade so that the blade tip angle will not change, or alternatively, rotating the doctor blade support beam about the blade tip edge by an incremental angle corresponding to the change in the blade angle. Such a constant-angle control is easy to implement in doctor blade units removed from the applicator apparatus, because the doctor blade support beam does not carry other equipment related to the coating process. In the short-dwell coating process, the use of blade-deflecting arrangements is awkward, because the required apparatuses are difficult to adapt in conjunction with the extremely compact construction of the short-dwell coater. Therefore, the blade angle in short-dwell coaters is simply controlled by rotating the doctor blade support beam about the edge of the blade tip. Also in this manner the control of the blade angle in short-dwell coaters is complex, because when the doctor blade support assembly is rotated about the edge of the blade tip, the gap of the coat metering edge of the applicator chamber to the web changes at the ingoing side of the web to the chamber. Obviously, this also changes the conditions in the applicator chamber and may permit the entry of air past the metering edge into the applicator chamber, which causes coat defects. Therefore, when doctor blade loading is changed in a short-dwell coater, the doctor blade support beam should also be rotated and simultaneously the coat metering edge adjusted, which operations are extremely cumbersome to implement and thus cannot provide a satisfactorily operating constant-angle doctor blade arrangement for a short-dwell coater. Major difficulties will also be encountered in designing the end dams, which control the coat width, so that they operarate compatibly with the blade tip angle changes.
However, the tip angle of the doctor blade to the web being coated can be held constant extremely well, independently of blade loading changes if the doctor blade is loaded close to its tip. Such apparatuses are already in use and, e.g., U.S. Pat. No. 4,440,105 discloses a short-dwell coater in which the doctor blade is loaded by means of a separate loading blade close to the doctor blade tip. The loading blade resembles the doctor blade in that it is a flexible blade pressed against the doctor blade by means of a resilient loading hose. While this design attains constant-tip-angle loading of the doctor blade with relatively good accuracy, it still has several drawbacks. During adjustment, the flexible blade slides along the rear surface of the doctor blade, and the friction causes uneven movement between the blades which disturbs blade control. Moreover, control of doctor blade profile by means of the flexible backing blade is difficult.